Three years ago Phil Kalas, former Horween Leather employee and current owner of Ashland Leather, sat down with Ben on the Shoecast to tell us (almost) literally everything we needed to know about leather.
Since then, we’ve found ourselves referring back to that interview, and Phil’s very helpful YouTube channel, for information about the tanning process. Now, appearing for the first time in written form, is an edited and expanded version of Phil’s crash course.
There are lots of tanning outfits that only handle one or two steps of the process below, or which only utilize a single technique for a given step. Horween does them all from start to finish, and has the equipment to carry out most steps in the process using an assortment of methods depending on the desired properties of the final tannage. That grants them a relatively rare birds-eye view of the process. Like any industry, tanning is constantly changing, and many tanneries juxtapose new technologies with techniques that have been in use in basically the same form for thousands of years.
This overview isn’t exhaustive, but it’s about as close as we could manage without opening a publishing company.
The Raw Hide: What, Exactly, Are The Raw Ingredients and Why Do They Matter?
Obvious but often overlooked: leather production always starts with an untanned hide. Tanneries generally receive whole hides packed folded on pallets and soaked in brine to prevent them from decaying on the way to the tannery. The size of these hides varies widely, but some can be up to 60 square feet or even larger.
The most common are bovine hides—in particular, steerhides. While “cowhide” is sometimes used as a catchall term, there’s a lot of variation between in bovine hides depending on how the animal spent its life. Technically, “cowhide” only refers to skins that come from female cattle. In our agricultural system, male cattle raised for foodstuffs are often castrated, in which case they’re referred to as “steers”. Otherwise, they remain “bulls”.
On average, cowhide is thinner, has a looser grain structure in the belly, and often has more markings. It’s rarely used for heavier applications. Bullhide is larger and thicker than either cowhide or steerhide, and often has heavily wrinkled shoulders. Of the three, steerhide is generally considered to be the most consistent. From there, things get a lot more specific—Horween usually purchases Heavy Native Steer (HNS). Hides represent only a small portion of the value of beef cattle and the USDA considers them a byproduct.
The next most common hide type that Horween uses is equine hides. Horses are still raised for their meat in some areas, and regulated industries still exist in Canada and Europe. In these areas, the industry is generally subject to the same or stricter regulations as the cattle industry, and ethical concerns primarily pertain to the transport of live animals to other areas, not domestic production. Other markets are less transparent. Horween primarly sources its equine hides from Europe, and states that all of the hides it tans are meat byproducts. Equine leathers are dense, strong, and usually thinner than steerhide. One of the most famous is Shell Cordovan, which can only be made with material that contains the fiber structure found on the hindquarters of all equine species.
Regardless of species, all of the processes that happen later on modify these raw hides in some way—they can become denser or more supple for example, or pigment can be added, but those modifications always work with what’s already there. A hide which naturally has very loose grain can be put through a series of processes identical to a naturally dense hide to produce an end result with some pretty noticeable differences. Of course, there’s lots of variation within each hide, from the even and dense back to the looser and more variable belly. That means two pieces of what is ostensibly the same leather might still be quite different!
The whole bovine hides received by the tannery are cut in half or “sided” to make them more manageable (a few tanneries will run whole hides), and then make their way to the first true step in the process.
De-Hairing and Curing: How Hides are Prepared for Tanning
Tanning is a very complicated chemical process where minutely small differences can be make or break—everything needs to be carefully controlled to ensure the process goes as planned. That starts with soaking, which is exactly what it sounds like: the hides are soaked in water to rise out the salt that preserved them during shipping, and then soaked again in a detergent solution to begin cleaning them of dirt, grime, organic debris, and anything else you don’t want in your leather.
Next, the hides are dehaired (though we have to admit it would be kind of cool if everyone was walking around in hair-on-hide boots all the time). The hides are placed in a large mixer, along with water and chemicals tailored to break down keratin (the protein your hair and nails are made of), and break the bonds between the hair and the skin. Left for too long, this would dissolve the hair and the hide into one thick goop. To prevent this, lime is applied to the mixer to interrupt the reaction, and then the mixer is drained and washed. Lime also “engorges” the leather, causing it to swell and pucker up so that it better accepts the compounds that will be introduced to it later.
Hides have two sides: the grain side has smooth, tight fibers and is dotted with tiny holes left by hair follicles, while the flesh side has a loose and fuzzy fiber structure. During the animal’s life, the grain side faced “out” while the flesh side connected the skin to the tissue and muscle underneath. Once all the hair is removed from the grain side, the hide is ready for “lime fleshing”. During this step, the flesh side has any remnants scraped off.
After that, the lime is washed out during de-liming in order to ensure that the ph is friendly to the next two steps: baiting and pickling. This is critical because baiting uses enzymes to break down parts of the skin that you don’t want. Leather is essentially a latticework of fibers—kind of like felt. Those fibers are mostly made up of collagen, but there are also several smaller proteins which interfere with tanning and contribute less strength. Baiting dissolves these proteins, allowing them to be washed away. Like many processes described here, it’s also a multifunctional reaction that we’d need an advanced degree to fully explain. One thing we can say as laymen: the hide is much more supple after this step.
The pickling process has a lot in common with pickling a cucumber (just skip the dill). The hides are soaked in a saline solution, causing them to swell up further. Swelling increases the surface area of the hide and opens its pores to help absorb tanning compounds. Because the tanning compounds are acidic, pickling adjusts the leather’s pH to prevent rapid changes that might cause damage. Leather is done pickling when it holds a thumbprint; if it springs back, that’s a sign that the core of the hide isn’t fully pickled. You can also check if a hide is tanned by biting it and waiting to see if it begins to dissolve on your teeth. Arnold Horween Jr. (Nick Horween’s grandfather) used to demo this method at the tannery, and supposedly it was common practice before the invention of pH test strips. Ahhh, the good old days.
Tanning: Bonding Natural Fibers with Tree Bark or Chrome
Almost a thousand words into our story about tanning, it’s finally time to talk about tanning.
Remember those fibers from earlier? On the flesh side of the leather, they’re especially soft and loose. (Phil Kalas imagines them as a bowl of spaghetti.) The goal of tanning is to fill in gaps and bind the fibers together. Imagine a woven cloth like duck canvas, made by long threads which pass over and under one another. If you cut holes in it, shortening the threads and creating exposed edges, it begins to fray and pull apart. The tanning process aims to take partially connected strands and turn them into a continuous web; it just takes place in three dimensions instead of two.
There are a lot of different compounds you can use to do that, but the most common are vegetable tannins and chromium salts: this step is the origin of the terms “veg-tan” and “chrome tanned”.
Chrome tanning is done in large drums—envision a wooden washing machine the size of a small house. Hides, water, and chromium salts are added to the drum, which rotates slowly, working tanning reagents into the hide. The product is durable and heat resistant, with (all else equal) more tensile strength than other leathers. Generally speaking, chrome tanned leathers are also softer and more flexible than their vegetable-tanned leather. Chrome gives the leather a blue cast, lending the product of this step its name: wet blue. Depending on how they’re finished, many chrome tanned leather will retain a blue core.
Vegetable tanning uses tannins derived from tree barks or other plant matter. Oak, chestnut, mimosa, and quebracho are all common species which impart slightly different properties to the end leather. Traditionally, vegetable tanning is done it pits. It takes longer, producing a firmer leather that burnishes and patinas dramatically.
Tree bark tannins, like other tanning compounds, are highly astringent. If you’ve ever drank tea that steeped for too long and felt your mouth dry up, that’s astringency. Chemically, it causes tissues to constrict, so during tanning the acidity and astringency have to be increased slowly to prevent the outer layer of the hide from absorbing the tannins, closing up, and leaving a “raw” middle that will rot. Shell Cordovan in particular is especially time intensive because of its density—eight to ten times that of calfskin.
While the vast majority of the leather you’ll come across is either chrome-tanned or veg-tanned, there are a number of other tanning reagents and variations on each process. Olive tanning, for example, is a relatively new vegetable tanning method that uses fallen olive leaves rather than tree barks. More drastically, a variety of synthetic tannins have been produces that approximate certain properties of vegetable and chrome tannages.
After tanning, the once perishable hide prone to rot and decay has become something persistent and durable.
Blue Sorting: What to Cover Up, and What to Show Off
Althought those wet blue and wet white hides are no longer at risk of rotting, they’ve also had all the natural fats and oils—which would otherwise lubricate them—removed. If they dry out now, they’ll become inflexible and brittle. Because of that, there’s a bit of a race against the clock to complete “blue sorting,” the allocation of hides to different final products.
Leather unsurprisingly scars in very much the same way your skin does, meaning nicks, cuts, and bug bites collected over the life of the animal are visible in the finished hide. There’s also “manure damage,” which is exactly what it sounds like—manure can chemically burn a hide, leaving areas of discoloration. On a black, embossed leather with a highly pigmented finish, that’s not a big deal because you’d never see them anyway. On an unfinished (“crust”) or very lightly finished leather, those marks might significantly reduce the useable area—and value—of the final product.
Of course this might still be pretty straightforward if there were no other constraints, but demand usually varies considerably across a tannery’s products. A large tannery like Horween accepts orders of varying sizes from a range of different companies across several industries; in Horween’s case footwear, sports, and small leather goods, among others. Grading requires an understanding of both the ever-shifting demand and the presently available supply in order to make case by case decisions about where to send each hide. Too many bad calls can put a tannery underwater.
Once sorted, leather then needs to be split down to the appropriate thickness. This is done on two machines: the splitter and the shaving machine. Splitters are analogous to planers in woodworking. A hide is run over a very sharp knife, creating two products: 1) one section at close to the desired thickness, which still contains the tighter and smoother grain layer, and 2) a “split” made up of the looser fibers on the flesh side of the hide. Splitting has produced a lot of misconceptions and marketing jargon, including the terms “top grain” and “genuine leather.”
“Genuine leather” technically refers to anything made out of real leather rather than faux leather, but its near-exclusive use marketing products made from heavily finished splits has given it a connotation of poor quality. “Top grain” refers to the part of the leather containing the grain (we’re detectives). This can be either “full grain” or “corrected grain,” meaning that the leather has been affected with sandpaper or a polishing cloth to smooth out the peaks and valleys of its surface. Fair warning though: spend enough time online and you’ll inevitably see the same terms defined differently.
One thing that’s really important is that the only thing these terms tell you is what cross section of the original skin you’re getting. Which means they aren’t a great way to assess the quality of a leather. Horween sells its splits rather than finishing them, but there are tanneries (C.F. Stead comes to mind) which tan extremely fine and high quality splits.
Once a hide is split, it’s taken to the shaving machine (more analogous to sandpaper than a planer), and the flesh side is shaved down to its final thickness, opening up the fibers to prepare the hide for later stages of the process.
Retan and Color: Making Leather Flexible and Colorful (or Nearly Anything Else) With Wet or Hot Stuffing
Leather’s flexibility comes from the fact that there are fats and oils lubricating the fibers, filling gaps, and allowing it to flex smoothly. Remeber, all of the original fats and oils were removed to prevent rawhides from rotting and deteriorating. Retanning is an opportunity to replace them with stable conditioning agents, as well as other additives that give any particular tannage its defining properties.
“Wet Stuffing” (distinct from hot stuffing, which we’ll talk about next) achieves this by soaking the hide in drums full of an emulsion of water soluble fats. Color is often added at this stage to produce a drum-dyed leather, as are additional tanning agents. Retanning a leather is an opportunity to combine the strengths of different tanning reagents, producing a combination tanned leather.
One famous example is Horween’s Chromexcel, which is chrome-tanned and then veg-retanned in order to give it additional body and allow it to be burnished. Its combination tannage is one of the reasons Chromexcel was selected for use in military footwear—exposed chrome salts in early competitors caused chemical burns. Modern chrome-tanned leathers don’t pose the same risks.
Beyond retanning and dying, nearly anything is possible at this stage. Various additives can make a leather water resistant, fireproof, or even UV resistant. The world is your leatheroyster here. But because so much differentiation happens during wet stuffing, many tanneries keep their formulas secret.
Some waxes that aren’t water soluble can’t be added during the wet stuffing process. These have to be melted down in steam-heated wooden drums and pounded into the leather in a process called hot stuffing. Hot stuffing drums use wooden pegs which impact the leather as it tumbles, encouraging it to accept more waxes and fats. The higher temperatures also make the leather more absorbent. Hot stuffing veg-tanned leathers is a risky process, because too much heat can “cook” the leather, causing it to weaken and crack. While steps like baiting and pickling or tanning (duh) are ubiquitous, hot stuffing is far less common. It’s a feature at Horween, used on Chromexcel and Shell Cordovan.
The fat liquors and wax blends of this stage might be likened to commercial leather polishes or conditioners. Certain ingredients might produce more shine, create a stiffer and less flexible leather, or make a leather essentially self conditioning. As with many things in life, the more time spent here, the better the result. But added time means added cost.
Wet and hot stuffing also change the leather’s color. Standard Chromexcel comes in brown and shades adjacent to brown. It’s veg-retanned and heavily hot stuffed with fats and waxes that have a brown cast. Cavalier uses fewer tanning reagents and isn’t as heavily hot stuffed, which allows it come in brighter and more vibrant colors. Like all things tanning, it’s an exercise in making the right tradeoffs for a leather’s use case.
Drying: Balancing Square Footage, Grain Texture, and Efficiency
So far we’ve done a lot of soaking the leather in stuff: detergents, enzymes, tannins, fats—you get the idea. Once reconditioned, the leather can be safely dried, and there are a few different ways to do that. Horween uses four, which also seem to be the most widely used methods.
Why so many? All of them produce changes to the leather that may be desirable or undesirable depending on the application, though many tanneries only have the equipment for one or two processes.
The simplest and most intuitive method is air drying, which works just like drying your laundry on a clothesline. This usually takes a little over a week, and over the course of that time the leather shrinks by several square feet. Air-dried leathers are often denser, and the end product is more easily stretched than pasted or toggled leathers.
Toggling uses clips to stretch the hide across a metal frame, preventing it from shrinking as it dries in order to maximize yield. On toggle-dried leather, the grain sits higher above the skin, producing what Phil calls a “fluffy” or “pillowy” effect. While toggled leather can be left to air-dry, it’s more efficient to run the frames through a large oven, reducing the drying time from days to hours.
Less common (but more whimsical) is pasting. The grain side of the hide is coated in glue and then literally pasted to a pane of glass. Like toggling, it prevents the hide from shrinking, but instead of producing raised, pillowy grain, it flattens and compacts the leather’s surface, leaving it smooth. One example of a pasted leather? Horween Latigo.
Finally there’s vac drying, which solves a very specific problem. As mentioned earlier, veg-tanned leathers dry out and crack when exposed to too much heat (around or above 120˚ F), which makes them difficult and expensive to dry. Using a vacuum dryer to create negative pressure allows water to evaporate at low temperatures and then sucks the water away. Vac drying also compacts the grain, but not quite to the extent that pasting does.
Once drying is complete, the leather is usable and we’ve officially crossed the “crust” line. Crust leathers are tanned and either left completely unfinished or simply softened before being shipped out. Because they’re unfinished, you can clearly see everything that’s been put into the leather so far, including the specific oils and waxes used, any color added during retaining, and natural grain and figuring of the hide, complete with all of its scars and scratches. Crust leathers are often preferred for applications where the leather will be finished or dyed at the point of use, or for truly drastic patina. Most leather however, goes on to be finished.
Finishing (Or Not): Shine, Color, and So Much More
Once the crust line has been crossed, we’re off to finishing, which is kind of the Wild West compared to the fairly structured requirements of the process so far. The first and most consistent step is staking, which feeds the leather through a staking machine (sometimes called a Melissa machine) where small vibrating pins soften it, improving the feel and allowing it to lay flat.
After that, it’s common to for leather to receive a “finish system”—a combination of dye, an adhesion coat, and a top coat.
Dyes come in a number of types, but one of the most important distinction is the presence and number of solid pigments. Aniline dyes, defined by the absence of solids, are a lot like wood stain. They add color, but are transparent enough that you can still see the grain underneath, which also means it’s harder to guarantee that the color is perfectly even.
Contrast that with a “full finish” that deposits solid pigments. These behave a lot like paint, covering up the grain and any inconsistencies to producing a flatter and more consistent color, which in some cases might look slightly artificial. In between, small amounts of solid pigments can be added to aniline dyes to produce semi-aniline dyes, and so on. Nearly any blend is possible. Work and casual boots tend to lean into more natural, aniline dyed leathers, while dress shoes are more likely to prioritize the even and consistent look of a more heavily finished leather. Dyes and stains can be sprayed on, brushed or rubbed in by hand, or applied by a large rubber roller which lays down a heavy layer of dye without any texture.
The next common step is an adhesion coat, which works a lot like paint primer. Varying the strength of the adhesion coat can change both the texture of the top coat and the way it wears over time. Phil thinks it’s likely that the texture of Alden’s Arabica Lux leather comes from applying a weak adhesion coat and then tumbling the leather, causing the top coat to flake off in chunks.
The top coat is applied last, and primarily affects the luster and texture of the final leather. There are shiny top coats, matte top coats, and top coats with additives for other properties like UV resistance.
Another common step finishing step includes running the leather through a plating machine. By using various plates, you can affect the leather with a mirror shine, a dull sandblasted effect, or an embossed pattern. “Hair cell” embossing mimics the pattern of the hair follicles in the leather’s natural grain in order to give heavily finished leathers a less artificial look (with the effects ranging from hyper-convincing to obviously fake depending on the pattern), but paisley, pebble grain, and scotch grain patterns are all commonly embossed as well. Glazing is a similar, but rarer process where the leather is repeatedly passed under a narrow piece of glass mounted to a reciprocating arm. This darkens, shines, and compacts the leather—Horween’s Shell Cordovan is glazed.
These five steps don’t come close to exhausting what can be done with finishing, which encompasses an broad array of nonlinear processes. But they do give you a good idea of what you’re likely to see on most leathers.
When finishing is complete, the finished “sides” are graded based on how much cuttable area they contain, which Horween measures based on the number of inside quarter panels that can be clicked from a given hide. Other tanneries and cuts have their own grading systems.
Conclusion: On Your Boots and In Your Pocket
If you’re reading this, you’re probably not running a tannery (if you are: let us know how we did!) But if you’ve used a leather wallet, worn a leather jacket, or own leather footwear, then some or all of these steps played the primary role in determining how it looked, felt, and aged long before it entered your hands.
Understanding how leather is made helps you parse the claims manufacturers make about their product, sure. But it also helps you understand why it behaves the way it does so that you can take care of it better.